Allied Supply (in Wisconsin...' sells to repair shops and music stores) keeps some "King" springs and some "Besson" springs. Both are about the same size as Yamaha springs, but both are considerably more stiff. They also stock Conn sousaphone springs, but you need enough space below the depressed piston for that many coils to compress to use those, and the Conn sousaphone springs are also larger diameter and (if your bottom caps feature spring depressions) may not fit in all bottom cap spring depressions. Your local store or shop should be able to add four of whichever to their next order, as there is a minimum order requirement.

Do piston springs in brass instruments, even in very old instruments, ever actually wear out provided they've not been "messed with" (stretched, cut, re-wound, etc.)? How much use can they generally take?

This is not a commentary on the original poster's request, just a couple questions that came to mind from reading the thread.

Do piston springs in brass instruments, even in very old instruments, ever actually wear out provided they've not been "messed with" (stretched, cut, re-wound, etc.)? How much use can they generally take?

This is not a commentary on the original poster's request, just a couple questions that came to mind from reading the thread.

Y'know, I've wondered the same thing myself about old springs. Bronze needle springs on old woodwind instruments tend to require tweaking to restore lost tension; it would seem logical that an old coil spring could lose some of its oomph, too.

Bronze is an extraordinary material. Stainless steel is as well. The only really crappy piston springs that I have ever encountered were brass, but even thick brass piston springs seem to last for a remarkably long time. The only piston springs that I have replaced on instruments were those which were monkeyed with by people who shouldn't have been monkeying with them, brass springs which were eaten up with lime, and Yamaha springs with the silly-and-pointless plastic coating failing, and me too impatient to peel the plastic away.